Ultraviolet fiber laser system

ABSTRACT

Laser master oscillator-power amplifier system for generating high pulse energy, high average power laser pulses in the ultraviolet 191.25-201.25 nm and 243-246.25 nm spectral ranges, and in the visible 450-537.5 nm spectral range with controllable pulse duration and pulse repetition rate employ a master oscillator seed laser operating in the infra-red spectral range, and a single series connected chain of hybrid fiber-bulk optical amplifiers coupled to a non-linear frequency conversion unit to convert the laser pulses to the ultraviolet and visible spectral ranges.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/588,426, filed Aug. 17, 2012, and claims the benefit of U.S.Application No. 61/524,884 filed Aug. 18, 2011, and U.S. Application No.61/524,436 filed Aug. 17, 2011.

BACKGROUND OF THE INVENTION

The invention relates generally to lasers and laser systems, and moreparticularly to fiber lasers operating in the ultraviolet (UV) spectralrange.

Ultraviolet laser sources having high power and high pulse energy withshort nanosecond pulse widths are required for many importantapplications such as laser material processing and medicine, e.g., eyesurgery. Presently, these applications primarily use excimer gas lasersbecause of the lack of other suitable laser sources. Excimer lasers canproduce laser emissions at several main wavelengths, such as nanometer(nm) wavelengths of 351 nm, 308 nm, 248 nm, 193 nm and 157 nm. Whilebeing reliable laser sources, excimer lasers are bulky, require periodicservice and, therefore, can have high ongoing costs of ownership. Inaddition high power excimer lasers often exhibit poor beam quality, usehigh voltage electronics and have low wall plug efficiency.

Fiber lasers are a relatively new type of laser source that are capableof delivering laser emission in a wide spectral range from infrared (IR)to UV. Fiber lasers can operate in continuous wave (CW) to ultra-shortpulse modes with output power levels exceeding tens of kilowatts (kW).Commercially available fiber lasers operate in pulse and CW regimes, andat fundamental operating frequencies with micrometer (μm) wavelengths inthe 1 μm, 1.5 μm and 2 μm ranges. Nonlinear frequency conversion can beused to generate higher order harmonics and shorter wavelengths. Oneapproach to obtaining UV spectral wavelengths in the 190-196 nm rangehas been to use a fiber laser operating at a fundamental wavelength inthe 1.5 μm range and nonlinearly convert the fundamental frequency toits 8^(th) harmonic. However, obtaining the desired average power leveland pulse energy for nanosecond pulses in the higher harmonic spectralemissions has been a problem.

Several nonlinear optical effects, mainly Stimulated BrillouinScattering (SBS), Stimulated Raman Scattering (SRS) and Self PhaseModulation (SPM), as well as the bulk optical damage in a fiber corelimit the pulse energy of a fiber laser. The fiber laser pulse energy isusually restricted to a fraction of a millijoule (mJ) for nanosecond(ns) pulse widths and the pulse peak power is restricted to tens of kW.

This peak power limitation may be overcome using ultra short pulsegeneration in the picosecond (ps) and femtosecond (fs) ranges due tohigher optical damage threshold of the glass fiber in these ranges oflaser pulse width. Ultra short pulse, high peak power fiber lasers cangenerate high average powers with close to transform limited pulses.However, even for transform limited operation, the ultra short pulsefiber lasers have relatively broad spectral line widths (this is a basicquantum mechanics uncertainty principle). For example, a 1 ps transformlimited Gaussian shaped optical pulse has approximately a 3 nm spectralline width at a 1.5 μm wavelength. This broad spectral bandwidth limitsefficient nonlinear frequency conversion to higher optical harmonics. Inaddition increasing the ultra-short laser pulse energy significantlyincreases the already high peak power which, in turn, triggers thedetrimental nonlinear optical effects that impact the fiber laser powerscaling.

Thus, power and pulse energy scaling of a fiber laser is a challengingtask, particularly when applications require close to diffractionlimited beam quality, a pulse width less than 10 ns, tens of watts ofaverage power, and a polarized beam. Scaling a fiber laser pulse energyin the visible and UV spectral range through nonlinear frequencyconversion is likewise limited by the low available pulse energy of thefundamental wavelength. With sub-mJ pulse energy at a fundamentalwavelength of 1 μm, the pulse energy in the 5^(th) or higher harmonicsis of the order of a microjoule (μJ).

One approach to overcome these problems and scale the power and pulseenergy to required levels in the UV range has been to use beam combiningarchitectures (fiber bundling) that combines the outputs of severalfiber laser sources to form a composite beam having increased averagepower and pulse energy with subsequent higher order nonlinear frequencyconversion to the UV spectral range. For example, MOPA (masteroscillator power amplifier) systems comprising erbium (Er) doped fiberssynchronously seeded by the same master oscillator (MO) operating in the1.5 μm range have been bundled to form a composite beam, andsubsequently converted to the 193 nm spectral range. While fiberbundling can increase the pulse energy in the fundamental beam and highorder harmonics, combining the outputs from bundled individual fiberlasers deteriorates the resulting laser beam and requires fine(nanosecond or sub-nanosecond) gating of the individual laser pulses tooverlap. Additionally, the polarizations of the beams have to be alignedfor efficient nonlinear harmonic generation which complicates spatialalignment of the individual fiber laser sources in the bundle. Anotherpossibility is to combine multiple laser beams that have already beenconverted by the nonlinear frequency conversion to the UV range.However, this approach also requires several individual fiber lasersources, and is bulky, complicated and costly.

Other approaches to achieve UV fiber laser operation use opticalfrequency mixing of different pulsed MOPAs having different fundamentalwavelengths, for example, one having a fundamental wavelength of 10YY nmand another having a fundamental wavelength of 15YY nm (or 21 YYnm), toprovide trains of optical pulses. The 10YY-nm pulses are frequencyquintupled to a wavelength of 213 nm, and the 15YY nm (or 21YYnm) pulsesare mixed with the 213 nm pulses to provide pulses having a wavelengthof 193 nm. The 10YYnm and 21 YYnm MOPAs include a fiber-laser and a bulklaser amplifier. However, this still requires two laser systems whichneed to be synchronized, properly triggered, and spatially overlapped innonlinear crystals.

It is desirable to provide efficient power and energy scalable hybridfiber laser/bulk solid-state optical amplifier systems capable ofyielding a high pulse energy scalable to over 10 mJ, high average powerscalable to over 100 W, output pulse widths controllable and variablefrom sub-ns to hundreds of ns pulse duration, and pulse repetition ratescontrollable from tens of Hz to over a MHz.

It is to these ends that the present invention is directed.

SUMMARY OF THE INVENTION

The invention affords new laser systems based on fiber laser technologyand bulk solid-state gain materials, which provide laser emission linesclose to the 193 nm and 248 nm spectral lines, power and pulse energyscaling with close to diffraction limited laser beam quality, reducedlaser volume, improved laser efficiency, controlled pulse repetitionrate, controlled pulse duration and reduced cost. Laser systems inaccordance with the invention can replace excimer lasers.

In one aspect, the invention affords laser systems operating in theultraviolet and visible light ranges that have high pulse energy, highaverage power, and controllable pulse width and pulse repetition rate byusing a hybrid fiber-bulk master oscillator-power amplifier systemcomprising a low power seed laser and a hybrid fiber-bulk opticalamplifier and a subsequent nonlinear frequency conversion unit. The bulkamplifier may be a laser amplifier, an optical parametric amplifier, oran optical Raman amplifier (or a combination thereof), as will bedescribed.

The invention further affords high power, pulsed, diffraction limitedYtterbium (Yb³⁺) doped fiber lasers operating in the spectral vicinityof 980 nm which corresponds to the zero-phonon emission line of Yb³⁺ inglass fiber between 972 nm and 985 nm in the near infrared spectralrange, with subsequent pulse energy amplification using a matchingoptical bulk amplifier and non-linear frequency conversion to thevisible region between 486 nm to 492.5 nm, the UV region of 243 nm to246.25 nm, and to the 194.4 nm to 197 nm spectral range. The bulkamplifier may be a laser amplifier, an optical parametric amplifier, oran optical Raman amplifier, as will be described.

The invention, in another aspect, affords a high power, pulseddiffraction limited Er³⁺ doped or Er³⁺—Yb³⁺ doped fiber laser systemoperating in approximately the 1550 nm spectral range corresponding tothe Er³⁺ emission line in optical fiber glass between 1530 nm and 1610nm in the near infrared region, with subsequent pulse energyamplification using a matching bulk optical amplifier and non-linearfrequency conversion to the 191.25 nm to 201.25 nm UV spectral range.The bulk amplifier may be a laser amplifier, an optical parametricamplifier, or an optical Raman amplifier (or a combination thereof), aswill be described.

The invention, in yet a further aspect, affords high power, pulsed,diffraction limited Thulium (Tm³⁺) doped fiber lasers system operatingin the 2 μm spectral range which corresponds to the Tm³⁺ emission linein optical fiber glass between about 1800 nm and 2150 nm in the nearinfrared with subsequent pulse energy amplification using a matchingoptical bulk amplifier and non-linear frequency conversion to the 450 nmto 537.5 nm blue-green spectral range. The bulk amplifier may be a laseramplifier, an optical parametric amplifier, or an optical Ramanamplifier (or a combination thereof), as will be described.

The invention, in still another aspect, affords simultaneous scaling ofthe laser pulse energy, and control of the laser pulse duration and thelaser pulse repetition rate using a matched hybrid fiber-bulk opticalamplifier comprising laser materials doped with an element selected fromthe group Yb³⁺, Er³⁺, Er³⁺—Yb³⁺, Tm³⁺ and Ho³⁺ (Holmium), and nonlinearfrequency conversion of the amplified pulses to produce laser lines inthe visible and UV spectral ranges. The bulk amplifier may be a laseramplifier, an optical parametric amplifier, or an optical Ramanamplifier (or a combination thereof), as will be described.

Applications of laser systems in accordance with the invention includematerials processing, laser communication, and laser medicine,especially for treatment of the eye.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic view of a generalized hybrid fiber-bulk lasersystem in accordance with an embodiment of the invention;

FIG. 2 is diagrammatic view of alternative embodiments of a fiberpre-amplifier system of FIG. 1 in accordance with an embodiment of theinvention;

FIG. 3 illustrates the absorption and emission spectra of Yb³⁺ in glass;

FIG. 4 is a diagrammatic view of a 245 nm fiber laser system inaccordance with a second embodiment of the invention;

FIG. 5 is a diagrammatic view of a 196.2 nm hybrid fiber-bulk lasersystem in accordance with a third embodiment of the invention;

FIG. 6 is a diagrammatic view of a 193 nm hybrid fiber-bulk laser systemin accordance with a fourth embodiment of the invention; and

FIG. 7 is a diagrammatic view of a 486 nm hybrid fiber-bulk laser systemin accordance with a fifth embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is particularly well adapted to providing high pulseenergy, high average power, and controllable pulse width and repetitionrate ultraviolet lasers for use in medical applications and the like,and will be described in that context. It will be appreciated, however,that this is illustrative of only one utility of the invention.

As will be described in more detail below, the invention employs bulkamplifiers for high energy power amplifiers. The bulk amplifiers may bebulk optical amplifiers.

There are two main types of optical amplifiers. One is based on thephenomenon of stimulated emission, and the other is based on opticalnonlinearities. Bulk solid-state optical amplifiers may employ differentsolid-state materials (doped or un-doped) as a gain medium. The mostwidely used bulk solid-state optical amplifiers employ optically excitedglasses, dielectric crystals or crystalline ceramics. Semiconductoroptical amplifiers (SOA), which demonstrate high gain, are not usuallyused in high energy bulk amplifier geometries and are typically fibercoupled. Because of their nature and design geometry (similar to laserdiodes), SOA is a good choice for high gain and low pulse energyamplification.

A bulk laser type amplifier is based on the phenomenon of stimulatedemission in optical centers (typically rare earth or transition metalions) doped bulk dielectric solids, usually crystalline or glass media.Laser active ions have an inversion population between certain systemsof energy levels. By injecting a seed light into such an inversionpopulated optical amplifier, the seed signal may exhibit netamplification using stimulated emission if the total gain overcomestotal loss in the medium. Typical examples of the laser type solid-statebulk amplifiers are Nd³⁺:glass (also used in a fiber amplifier),Er³⁺—Yb³⁺:glass (also used in a fiber amplifier), Yb³⁺:glass (also usedin a fiber amplifier), Tm³⁺:glass (also used in fiber amplifier), Nd³⁺:YAG or Nd³⁺:YAG ceramic, Ti³⁺:Al₂O₃, etc. The most effective solid-statelaser amplifiers are diode lasers optically pumped or pumped by othertypes of laser sources. Among several important characteristics of laseramplifiers, one of the most important is their capability to storeoptical energy. This feature fundamentally differentiates them fromnonlinearity based optical amplifiers. Unlike laser active ion-dopeddielectric solids, SOA, which is also based on the phenomenon ofstimulated emission and inversion population, has a very limited energystorage capacity.

Typical examples of optical amplifiers based on intrinsic opticalnonlinearities are Raman amplifiers (RA), Brillouin amplifiers (BA) andoptical parametric amplifiers (OPA). RA and BA use the coherentinteraction of the pump and seed optical beams with acoustical phonons(as in the case of RA) or optical phonons (as in the case of BA) in theamplifier media through stimulated Raman scattering (SRS) and Brillouinscattering (SBS) processes, respectively. During these processes thepump beam either adds or gives up energy equal to the energy of theeffective phonon (acoustical or optical) in the medium. When this phononenergy is added to the pump photon energy, the resulting photon iscalled an anti-Stokes photon. When the phonon energy is subtracted fromthe pump photon energy, the resulting photon is called a Stokes photon.Injecting into the optically excited RA a seed beam with photon energyequal to the Stokes or anti-Stokes photon energy may result in netoptical amplification at the output of the amplifier if the Raman gainovercomes the total loss in the Raman medium.

During an optical parametric amplification (OPA) process, the gainmedium demonstrates a coherent interaction of pump and seed opticalbeams, producing a third optical beam which satisfies the following twoconditions: 1) energy conservation for all three photons participatingin the process, and 2) conservation of momentum of all threeparticipating beams. During the OPA operation, the pump photon gives upcertain energy which is split between so-called signal and idlerphotons. The newly created signal and idler photons satisfy theconditions of the energy and momentum conservation, as described above.Most practical applications utilize OPA or RA type optical amplifiersbased on intrinsic optical nonlinearities. Typical examples of the OPAtype solid-state bulk optical amplifiers are crystals which demonstratesignificant optical parametric gain, namely Beta Barium Borate β-BaB₂O₄(BBO), Bismuth Triborate BiB₃O₆ (BiBO), Potassium Titanyle ArsenateKTiOAsO₄ (KTA), KTiOPO₄ (KTP), Potassium Dihydrogen Phosphate KH₂PO₄(KDP), Deuterated Potassium Dihydrogen Phosphate (DKDP), LithiumTriborate LiB₃O₅ (LBO) etc. Many quasi-phase-matched crystals, such asperiodically poled (i.e., PP) MgO:LiNbO₃ (MgO-PPLN), periodically poledstoichiometric Lithium Tantalate (PPSLT), MgO doped PPLT, MgO dopedStoichiometric PPLT, PPRTA, PPKTP and others also demonstrate opticalparametric gain and are widely used as an active medium for opticalparametric oscillators (OPO) and optical parametric amplifiers (OPA).Typical examples of the RA type solid-state bulk optical amplifiers arecrystals which demonstrate significant Raman gain, namely Ba(NO₃)₂(BNO),KY(WO₄)₂ (KYW), KGd(WO₄)₂ (KGW), CaCO₃, diamond, silicon and others.Unlike laser type amplifiers, optical amplifiers based on intrinsicnonlinearities fundamentally do not demonstrate optical energy storageand provide optical amplification only during the presence of the pumpbeam.

Taking into account the above description of existing bulk opticalamplifiers, the types of the bulk solid-state amplifier that may be usedin the invention are broad. For the invention, the bulk amplifier may beeither a laser amplifier, an optical parametric amplifier, or an opticalRaman amplifier. Depending on the particular situation and design, thebulk amplifier may comprise of a chain of different types, sequences andquantities of the mentioned above bulk amplifiers. For example, the bulkamplifier may comprise one laser amplifier followed by an opticalparametric amplifier. The bulk amplifier may also comprise one singletype solid-state amplifier or multiple types of bulk amplifiers inmultiple quantities.

FIG. 1 illustrates a generalized fiber laser system 100 in accordancewith the invention. As shown the system may comprise a single seriesconnected chain comprising master oscillator (MO) 110 operating in a CWor pulse mode, a fiber amplifier system 112, and a bulk solid-stateoptical amplifier system 138 comprising one or more stages coupled to anonlinear frequency conversion unit (NLFCU) 106. The bulk amplifier maybe a laser amplifier, an optical parametric amplifier, or an opticalRaman amplifier (or a combination thereof), as previously described.Fiber amplifier system 112 may comprise a fiber laser pre-amplifiersystem 102 and a power amplifier (PA) system 104 which may comprise oneor more amplifier stages. System 100 further comprises various opticalsystem components which link the MO and individual sections of the PAincluding components for temporal and spectral shaping of the laserpulses, spatial mode matching, and optical isolation to protect the PAsection from detrimental nonlinear effects created in other sections orfrom feedback of the external to the laser system optical components.Similarly, optical isolators 126 may be installed between poweramplifier system 104 stages, and a bulk optical amplifier system 138 canbe used to protect power amplifier system from back reflectionoriginating from either focusing optics 136, bulk amplifier system 138or nonlinear frequency conversion unit 106. The bulk amplifier may be alaser amplifier, an optical parametric amplifier, or an optical Ramanamplifier (or a combination thereof), as previously described.

The master oscillator 110 may be coupled to the fiber amplifier system102, two embodiments of which (102A and 102B) are shown in FIG. 2. For alow power MO (optical power density out of the MO fiber core attenuatedby the intensity modulator duty cycle is much less than the gainsaturation intensity of the next fiber amplification stage) theinvention may use fiber amplifier system 102A, (FIG. 2), to amplify theMO emission before intensity modulation. As shown in FIG. 2, the fiberamplifier 102A may comprise an isolator (ISO) 120, followed by a core orcladding pumped fiber amplifier (GF) 114, an intensity modulator 116, aband pass filter (BF) 118 and optical isolator 121, connected in series.If the MO optical power is high (optical power density out of MO fibercore attenuated by the intensity modulator duty cycle is close to thegain saturation intensity of the next fiber amplification stage) theinvention may use the embodiment of the fiber amplifier system 102B asshown in FIG. 2 to amplify the MO emission after intensity modulation.The components of the fiber amplification stages are preferably coupledtogether by polarization maintaining (PM) optical fiber 128. The MO 110may be either pulsed or continuous wave (CW) with a spectral emissionband in the gain band of the fiber amplifiers 114, 122 and bulk opticalamplifier 138. In the case of a pulsed MO 110, the intensity modulator116 is not required. The bulk amplifier may be a laser amplifier, anoptical parametric amplifier, or an optical Raman amplifier (or acombination thereof), as previously described.

The gain optical fiber used in the amplifier 114 as a gain medium, maybe of different types and structures, such as: (1) a polarizationmaintaining (PM) single clad, core-pumped Yb³⁺, Er³⁺, Er³⁺—Yb³⁺ or Tm³⁺doped glass fiber; (2) a polarization maintaining (PM) cladding pumpedlarge mode area (LMA) Yb³⁺, Er³⁺, Er³⁺—Yb³⁺ or Tm³⁺ doped claddingpumped double clad (DC) or cladding pumped triple clad (TC) glass fiber.A LMA fiber is a fiber with a relatively large mode area core whichsupports a single transverse propagating mode or a few such modes. A LMAfiber may be produced by creating a core with a numerical apertureNA<0.1, i.e., a condition where the refractive index difference of thecore and cladding is decreased (as in a step index fiber design) so thatthe core propagating mode fields penetrate into the cladding where theirpropagation loss increases with bending. This effect may be used tofilter or attenuate high order spatial fiber modes by coiling the LMAfiber with a certain radius of curvature leaving only the fundamentalpropagation mode with close to diffraction limited beam quality. Thetypical core NA of the commercially available LMA fibers is in the range0.06-0.08, and LMA fiber core diameters are typically of the order of10-30 μm. SOA can also be used in the fiber amplifier stages.

Seed laser 110 may comprise one of the following types of singlefrequency (SF) MO lasers: a) any external cavity diode laser (ECL) whichuses a diffraction grating as an output coupler and wavelength locker,external cavity diffraction grating based diode laser cavities in aLittrow or Littman configuration including tunable lasers, or a fiberBragg grading (FBG) locked based diode laser or fiber laser; b) adistributed feedback diode laser (DFB); c) a distributed Bragg reflectordiode laser (DBR); d) a fiber laser (FL) in either a linear or ringlaser cavity or which employs distributed feedback; e) a microchip orother diode pumped solid state (DPSS) lasers with or without Q-switchand/or mode-lock modes of operation, or f) a semiconductor opticalamplifier (SOA) based laser sources including linear or ring fibercoupled SOA based fiber lasers.

When a continuous wave (CW) MO is employed, the intensity modulator 116may create optical pulses for subsequent amplification. The intensitymodulator in this case may be, for example, a) an acousto-opticalmodulator (AOM); b) a Mach-Zehnder modulator (MZM) including small chirpor zero chirp modulators; c) an electro-optical modulator (EOM); or d)an attenuated total internal reflection modulator (ATIRM). Asemiconductor optical amplifier (SOA) can also be used as a gainmodulating device for a CW propagating beam in an SOA. An optical gainmodulated SOA, in addition to creating pulses, may produce chirp free orreduced chirp pulse operation.

When a pulsed MO is desired, direct current modulation of thesemiconductor laser output power may be employed using a pulse laserdiode controller. In addition, current modulation of the diode laser canbe used to create a mode-lock, Q-switch or hybrid Q-switch-mode-lockoperation of the laser diode. Passively Q-switched, passivelymode-locked (or a combination of both) DPSS lasers including microchiplasers and fiber lasers either based on ring or linear laser cavitiesmay be used as a MO.

The MO 110 may be either a linear or ring laser cavity employing asingle mode (SM), polarization maintaining (PM) or double clad (DC) SMor PM gain fiber. The gain fiber may be located inside a laser resonatorcomprising two or more mirrors, dispersive elements for wavelengthselection/tuning, or just a closed loop of fibers and/or other opticalcomponents to create a ring laser cavity (including Q-switches andmode-locking devices).

The power amplifier system 104 can be a single or multi-stage CW orpulse pumped optical fiber amplifier (as shown in FIG. 1, for example)having an optical gain band overlapping the MO seed laser 110 spectralemission line. Each fiber power amplifier stage may comprise a core orcladding pumped amplifier 122, a band pass filter 124, an isolator 126and an un-doped fiber 128, and is preferably separated by opticalisolator 126 to protect a preceding amplification stage from powerfeedback originating in the next amplification stage. In the case of thefirst amplifier stage, isolator 120 at the input of the pre-amplifier102A, 102B protects the MO 110 the isolator 121 at the output of thepre-amplifier 102A, 102B protects the fiber pre-amplifier system fromback reflections. If fibers with different core diameters are used inthe stages, mode field adapters (MFA) may be used to match mode sizes ofneighboring amplifier stages.

Band pass filters BF 118, 124 (FIGS. 1 and 2) may be used to filter outamplified spontaneous emission (ASE) from a main laser spectral line toachieve higher spectral purity and reduce noise of the subsequentlyamplified laser beam. Typical ASE suppressing devices includedielectrically coated narrowband filters which are BF, Fabry-Perotetalons, acoustic-optical modulators and volume holographic filtersbased band-pass filters.

The optical gain fiber used in the power amplifier 122 as a gain medium,may be of different types and structures, such as: (1) a polarizationmaintaining (PM) single clad, core-pumped Yb³⁺, Er³⁺, Er³⁺—Yb³⁺ or Tm³⁺doped glass fiber; (2) a polarization maintaining (PM) cladding pumpedlarge mode area (LMA) Yb³⁺, Er³⁺, Er³⁺-Yb³⁺ or Tm³⁺ doped claddingpumped double clad (DC) or cladding pumped triple clad (TC) glass fiber.The optical gain fiber used in fiber amplifier 122 may also comprise anyof: 1) a polarization maintaining (PM), i.e., single mode (non-LMA)Yb³⁺, Er³⁺, Er³⁺-Yb³⁺ or Tm³⁺ doped cladding pumped double clad (DC) orcladding pumped triple clad (TC) glass fiber; (2) a polarizationmaintaining (PM) photonics crystal fiber (PCF) Yb³⁺, Er³⁺, Er³⁺—Yb³⁺ orTm³⁺ doped cladding pumped double clad (DC) fiber; (3) a polarizationmaintaining (PM) rod type photonics crystal fiber (RPCF) Yb³⁺, Er³⁺,Er³⁺-Yb³⁺ or Tm³⁺ doped cladding pumped double clad (DC) fiber; (4) apolarization maintaining (PM) rod type single crystal fiber (RSCF) RE³⁺doped gain fiber or (5) a polarizarion maintaining (PM), coilablecrystalline RE³⁺ doped gain fiber.

The optical gain fiber used in the power amplifier 122 is preferablyheat sinked for heat removal. A heat sink may be formed of acylindrically shaped component preferably made of a high thermalconductive metal such as, but not limited to, copper or aluminum (orother metals), having spiral grooves about its surface into which thefiber may be positioned and coiled about the cylinder. Additionally, atemperature gradient may be applied to the gain fiber coil to suppressstimulated Brillouin scattering (SBS) in the coiled fiber. Packaging ofthe fiber may also include thermally conductive semiconductor ordielectric materials such as but not limited to Si, alumina or ceramic.

When a LMA gain fiber is used in the fiber power amplifier, the fibermay be coiled to a certain radius of curvature to filter out higherorder spatial modes propagating inside the fiber and to afford close toor diffraction limited light emission from the fiber. Spatial modefiltering may be implemented by using the heat conductive cylindricalheat sink (with active or passive heat removal) described above aroundwhich the fiber is coiled. The cylinder can act simultaneously as a bothspatial mode filter and a heat sink.

To reduce chirp in propagating pulses, a chirped FBG or fiber coupledchirped volume holographic grating (VHG) may be included. Additionally,an inter-amplifier device such as a power coupler or directional couplerterminated with a photo-detector for power or pulse monitoring (notshown) may be included.

Optical power (or pulse energy in the case of pulse operation) enteringeach amplification stage is preferably brought to the level required forgain saturation in the amplifier for best power conversion efficiency(i.e., amplifier stored energy extraction efficiency) and reducedamplified spontaneous emission in the amplifier. This may beaccomplished with additional pre-amplifier stages as well as multi-passamplification geometries. Pre-amplification system 102 which may includean optical isolator 120 and band pass filter 118 may be used, forexample, for pulsed laser operation (either using direct currentmodulation of the seed diode laser or CW seed laser output intensitymodulation). Pulse operation reduces the average output power to a leveldepending on the pulse duty cycle. Reducing the average optical power ofthe pulsed MO may require using one or more pre-amplifier stages 102 (anintensity modulator is required only in one pre-amplifier stage) beforethe high power, i.e., booster fiber amplifier system 104.

The bulk optical amplifier 138 may be an excited laser gain crystal, alaser active crystalline ceramic, a laser active glass, an opticalparametric amplifier or an optical Raman amplifier (or a combinationthereof, as described above) having a net optical gain and a goodoverlap between the fiber laser fundamental wavelength and the bulkamplifier gain band (preferably corresponding to the maximum of theamplifier gain band or within full width half maximum (FWHM) of theband). When the bulk amplifier comprises a laser amplifier, the bulklaser amplifier may comprise laser active crystalline materials such asdoped with trivalent rare-earth (RE³⁺) elements or trivalent ortetravalent transition metals (TM³⁺ or TM⁴⁺) and is preferably pumpeddirectly by laser diodes or diode pumped solid-state laser pumpedanother solid-state laser amplifier such as a Ti:sapphire amplifierwhich can be pumped by a green DPSS laser. The bulk optical amplifiershould match the MO and the fiber amplifier system. As used herein,matching means that the bulk amplifier comprises one or more types ofoptical amplifiers and one or more stages of each type of opticalamplifiers that a) have an optical gain which corresponds to spectralwavelength of the MO, b) the whole system of optical amplifiers iscapable of producing at least 0.5 mJ of pulse energy and at least 0.005W of average power simultaneously, and c) operate in a pulse width rangebetween 0.1 ns to 10,000 ns. When the bulk amplifier comprises anoptical parametric amplifier, the bulk optical parametric amplifier maycomprise either critically phase-matched crystalline nonlinear opticalmaterials or quasi-phase-matched crystalline optical materials. The bulkoptical parametric amplifier is preferably pumped by a solid-state laser(or its optical harmonics), such as DPSS laser including injectionseeded solid-state lasers. Similar to what is described above for lasertype bulk amplifier, the bulk optical parametric amplifier should matchthe MO and the fiber amplifier system. As used herein, match means thatthe bulk optical parametric amplifier comprises one or more opticallypumped nonlinear optical, parametrically active crystals that a) have anoptical gain which corresponds to spectral wavelength of the MO, b) incombination of other used types (if any) of the bulk amplifier togetherare capable of producing at least 0.5 mJ of pulse energy and at least0.005 W of average power simultaneously, and c) operate in a pulse widthrange between 0.1 ns to 10,000 ns.

When the bulk amplifier comprises an optical Raman amplifier, the bulkoptical Raman amplifier may comprise a crystalline nonlinear opticalmaterials demonstrating Raman gain. The bulk optical Raman amplifier ispreferably pumped by a solid-state laser (or its optical harmonics),such as DPSS laser. Similar to what is described above for laser andoptical parametric type bulk amplifiers, the bulk optical Ramanamplifier should match the MO and the fiber amplifier system (or anyother predeceasing bulk optical amplifier system). As used herein, matchmeans that the bulk optical Raman amplifier comprises one or moreoptically pumped nonlinear optical, Raman active crystals that a) havean optical gain which corresponds to spectral wavelength of the MO, b)in combination with other used types (if any) of the bulk amplifiertogether are capable of producing at least 0.5 mJ of pulse energy and atleast 0.005 W of average power simultaneously, and c) operate in a pulsewidth range between 0.1 ns to 10,000 ns.

In the case of a laser type bulk solid-state amplifier, the bulk laseramplifier material should preferably have high energy storagecapability, high thermal conductivity, high thermal shock tolerance andgood thermo-optical properties to afford multi-watt to over a hundred ofwatts of output power. The bulk solid-state laser amplifier may comprisea crystalline (including crystalline ceramic) or glass rod, disk or slabshaped laser amplifier elements with un-doped parts diffusion bonded oroptically contacted with the largest surfaces of the bulk amplifieractive element. The diffusion bonded or optically contacted parts areused for wave-guiding and/or heat management purposes in the bulkamplifier. Possible materials for the diffusion bonded or opticallycontact parts are un-doped version of the used bulk laser amplifiermaterial, sapphire, YAG, Silicon, other semiconductor materials whichdemonstrate high thermal conductivity. High laser gain will allow thebulk amplifier to use a minimum number of amplification passes (ideallysingle or double pass amplification geometries) for efficient energyextraction. Although laser active glasses demonstrate poor thermalconductivity, they can still be used in a low power pre-amplifier stagesof the bulk optical amplifier system, particularly when it is importantto amplify the laser pulse energy while not exceeding average powerbeyond the thermal destruction of the laser glass (typically severalhundred milliwatts).

High peak power pulses (or CW beam) exiting fiber amplifier 104 into thebulk solid state amplifier section 138 which may comprise 1 to Namplification stages get further coupled into the nonlinear frequencyconversion unit (NLFCU) 106. The bulk amplifier may be a laseramplifier, an optical parametric amplifier, or an optical Ramanamplifier (or a combination thereof), as previously described. Amplifiedin the bulk amplifier, the optical beam is coupled into a nonlinearcrystal 140 for second order harmonic generation (SHG). The NLFCU mayfurther comprise a second nonlinear crystal 141 for 3^(rd) orderharmonic generation (THG), third nonlinear crystal 142 for 4^(th) orderharmonic generation (FHG); and a nonlinear optical crystal or crystals144 for NthHG, i.e., higher orders of frequency conversion in the UVspectral range. Mixing fundamental IR radiation with SHG beam in anonlinear crystal 141 can create third order harmonic generation (THG),and fifth order harmonic generation (5HG) using another nonlinearcrystal. Higher order optical harmonic generation (HG) may be achievedby further mixing of beams produced through different frequencyconversion stages. In addition, individual stages of the NLFCU mayinclude one or more mirrors, beam splitters, different prisms such as aPellin Broca prism, and different types of diffraction gratings oroptical filters to separate different laser spectral lines. The NLFCUmay comprises a matching optical compensator prisms used in combinationwith an angle phase matched nonlinear crystals to preserve the initialbeam axis direction after the nonlinear frequency conversion.

The nonlinear frequency conversion through second order harmonicgeneration (SHG) may be implemented using nonlinear optical crystalssuch as (but not limited to) periodically poled (i.e., PP) MgO:LiNbO3(i.e., PPLN-MgO, i.e., doped with MgO), Periodically Poled PotassiumTitanyl Phosphate (PPKTP), Periodically Poled Rubidium Titanyl Arsenate(PPRTA), Periodically Poled Lithium Niobate (PPLN), PPLN doped withother metal ions or their oxides for improving laser power tolerance(i.e., increasing damage/degradation or darkening threshold),periodically poled Lithium Tantalate (PPLT), periodically poledstoichiometric Lithium Tantalate (PPSLT), LBO and other critically andnon-critically phase-patching nonlinear crystals.

The nonlinear frequency conversion through third, fourth or fifth orderharmonic generation (THG, FHG or F5HG) can be implemented in differentways, including but not limited to periodically poled (i.e., PP)MgO:LiNbO3 (PPLN-MgO) crystals working in the first or higher order ofpoling period/grating, PPKTP, PPRTA, PPLN, PPLN doped with other metalions or their oxides for improving laser power tolerance (i.e.,increasing damage or darkening threshold), periodically poled LithiumTantalate (PPLT), periodically poled stoichiometric Lithium Tantalate(PPSLT), MgO doped PPLT, MgO doped Stoichiometric PPLT, PPLT doped withother metal ions or their oxides for improving laser power tolerance(i.e., increasing damage or darkening threshold), operating in the firstor higher order of poling period/grating. Other nonlinear crystals canbe used such as Potassium Dihydrogen Phosphate (KDP), Deuteratedpotassium dihydrogen phosphate (DKDP), Beta Barium Borate (BBO), BismuthTriborate (BiBO), Cesium Lithium Borate (CLBO), Cesium Borate (CBO),Lithium Triborate (LBO) KBBF (KBe₂BO₃F₂), SBBO (Sr₂Be₂BO₇) etc., may beused.

In a preferred embodiment, the invention employs Yb-doped fiber lasers,and may use any Yb-doped fiber which demonstrates laser gain in thevicinity of the 980 nm emission band, i.e., the high energy luminescenceband of the ²F_(5/2) ²→F_(7/2) optical transition of Yb³⁺ in glass.

FIG. 3 illustrates the emission and absorption spectra for Yb³⁺-dopedglass. As shown, the highest absorption peak as well as the highestemission peak is A at 976 nm, and this wavelength is the most efficientpump light absorption for laser operation in the 1 μm spectral range.FIG. 3 also shows that Yb³⁺-doped glass not only has a high level ofabsorption at the 976 nm zero-phonon line (ZPL), but also demonstratesstrong emission at this wavelength with a high emission cross section.This has a dual impact on the optical and laser properties of Yb³⁺-dopedglass in the vicinity of its zero-phonon transition, i.e., near 976 nm.On one hand, laser action at 976 nm may be very intense, while on theother hand strong absorption, i.e., loss, at this same wavelengthcreates challenges in achieving population inversion and high net gain.Pumping Yb³⁺-doped glass fibers in a higher energy absorption part ofthe spectrum, i.e., at approximately 915 nm, produces strongluminescence in the 960-1100 nm spectral range. Pumping an Yb³⁺ opticalcenter in glass fiber near 915 nm and achieving laser emission in1020-1100 nm spectral range is described very well by a quasi-fourenergy level mode of the laser operation. However, if the same Yb³⁺laser center in glass is pumped near a 915 nm wavelength but produceslaser emission in the vicinity of the 976 nm wavelength, the laseroperates on a quasi-three energy level scheme.

One of the biggest difficulties in achieving laser action in lasershaving active centers which follow three-level operational scheme isthat the laser medium, as a condition to achieving laser threshold,requires conversion of over half of its particles from ground energylevel to an upper energy level. This is true for Yb³⁺-doped fiber laseroperation at 976 nm wavelength, i.e., at its zero-phonon line. The pumppower requirement translates into higher laser threshold compared tofour level lasers (such as Yb³⁺ doped fiber lasers operating in1020-1100 nm range) and requires higher pumping power density.Therefore, most efforts have been focused on core pumped a Yb-dopedfiber laser operating in the spectral range of its ZPL near 976 nm usinghigh brightness laser sources (single mode pump diode lasers,solid-state lasers or fiber lasers), because their radiation can beeffectively coupled into the core of the gain fiber. This approach,however, is costly and has limited power scaling capability. Otherapproaches have included utilizing a photonics crystal Yb-doped gainfiber or pumping the Yb-doped gain fiber laser medium with a high powersolid-state laser source (such as another fiber laser or diode-pumpedsolid-state laser), but they are complicated and costly. As will bedescribed, the invention solves these problems using all glass Yb-dopedfiber laser sources pumped by a low brightness diode lasers withnonlinear frequency conversion to the blue-green and UV spectral rangeallowing significant power scaling in the spectral vicinity of the ZPLof Yb³⁺, making them attractive for important applications such asoptical communication, material processing and medicine.

In one embodiment, the invention utilizes a fiber laser, either pulse orCW, based on Yb³⁺-doped gain fibers which operates in a gain band in thevicinity of the Yb³⁺ zero-phonon transition (ZPT) in the 960-1000 nmspectral range, (preferably 972-985 nm), as shown in FIG. 3. The fiberlaser may operate on its fundamental frequency or, after nonlinearfrequency conversion, in the visible and UV spectral range. Using acladding pumped LMA Yb-doped fiber, the fiber glass core with a corearea S_(core) and a NA<0.1 may be surrounded by an un-doped, high NA(typically NA≧0.4) glass inner cladding with a cladding area S_(clad).

The S_(clad) to S_(core) ratio of the fiber is chosen so that theoptimum gain fiber length preferably satisfies the following conditionssimultaneously: 1) more than 5% optical conversion efficiency from pumplight typically in spectral range between 900 and 930 nm to anoscillating line typically in spectral range between 973-985 nm; 2) aratio of the laser emission power in the spectral range 973-985 nm tothe power of the laser oscillating in the spectral range 1010-1100 nmhigher than one; 3) saturation of signal re-absorption transition undera given pump level and pumping geometry; 4) enough cladding opticalabsorption of Yb³⁺ ions at the pumping wavelength to achieve a minimumnumber of loops of the coiled gain fiber in order to filter out higherorder fiber spatial modes and leave the lowest spatial fiber propagatingmode; and 5) allows cooling the gain fiber in the coiled gain fibersection if required at the given level of pump power. As describedabove, a metal cylinder about which the fiber is coiled may be usedsimultaneously as a mode filter and heat sink, and it may be either aircooled or attached to the Thermo-Electric Cooler (TEC) plate or watercooled heat sink. A triple clad (TC) gain fiber can also be used. The TCfiber is preferably a large mode area (LMA) one that has a Yb dopedcore, i.e., a NA<0.1.

The Yb-doped core gain fiber preferably has the minimum possible Yb³⁺concentration (preferably less than 600 dB/m of the core absorption at976 nm in the case of double clad Yb³⁺-doped cladding pumped fiber) andminimum possible length, preferably to satisfy the following conditionssimultaneously: a) because of the three energy level operation of theamplifier/laser, pumping each section of the fiber length is preferablyimplemented with enough pump power to saturate absorption of the activeYb³⁺ centers at the oscillating wavelength to reduce re-absorptioneffects and losses to the laser radiation; and b) the fiber length ispreferably be long enough to coil the LMA fiber (one or more loops) tofilter out higher order spatial propagating modes and leave only thefundamental fiber propagating mode. Some LMA fibers like 10/125,NA_(core)=0.075, may not require coiling when used for guiding lightwith a wavelength close to 1 μm due to natural spatial mode filteringwith a low NA core.

A Yb-doped fiber in accordance with the invention may be made of anytype of polarization maintaining (PM) LMA fibers, e.g., including butnot limited to Panda geometry, bow tie etc., or may be a true singletransverse supporting mode LMA type fiber.

The Yb-doped DC or TC gain fiber (of any type listed above including aLMA) used either in the fiber amplifier or the pre-amplifier section ofthe system is preferably cladding pumped, i.e., the inner cladding ofthe fiber which surrounds the Yb-doped fiber core is optically pumped inthe spectral range corresponding to the high energy absorption band ofYb in glass between about 900 and 940 nm depending on the glass type.Optical pump sources are preferably fiber coupled multimode, lowbrightness diode lasers. The optical pump power coupled into thecladding of the DC or TC Yb-doped fiber or coupled into the core of thesingle clad Yb-doped gain fiber should be high enough to create anoptical power density sufficient to essentially reduce re-absorptionloss (because of quasi three-energy level operation of Yb-doped gainfiber when it operates in the spectral range of its ZPL near 976 nm) forthe seed light propagating inside the gain fiber core. The pumpwavelength is preferably in the spectral range of 850 nm to 950 nm whichcorresponds to the high energy absorption band mentioned above.

The output end of the Yb³⁺ fiber amplifier may be angle polished orangle cleaved to prevent back reflection of laser oscillation outputfrom the fiber end, which can restrict power scaling of the fiberamplifier. The output end of the Yb fiber amplifier may also beend-capped to increase power tolerance of the output fiber end to theoutput amplifier beam when a non-end-capped fiber output restricts laserpower scaling because of optical and thermal damage. End-cappingcomprises splicing a short length of a “coreless” fiber with thediameter usually close to the fiber cladding diameter to the end of thesingle mode, LMA or PM fiber. The end cap allows the light to expand indiameter before it emerges from the glass into air, reducing the powerdensity at the glass/air interface. Because the glass/air interface isthe most sensitive to damage, expanding the beam increases the damagethreshold of the fiber end.

A Yb-doped DC LMA gain fiber that is cladding pumped in the 915 nmabsorption band, (as described), may serve as an amplifier or apre-amplifier stage of the master oscillator-power amplifier (MOPA)Yb-fiber laser system, when the original seed laser source is either afiber laser operated in the spectral range between 973 nm and 985 nm ora diode laser (including PM and non-PM fiber coupled diode lasers) withoscillation wavelength in the spectral range between 973 nm and 985 nm,or in the gain bandwidth of the particular DC LMA Yb-doped fiber used.The seed laser of the MOPA system may be a single frequency DFB or anyother single frequency diode laser. A single frequency solid-statemicrochip laser may also be used. For greater line-width operation,Fabry-Perot diode lasers, other solid-state lasers, or tunable fiberlasers may be used.

Unused pump radiation either transmitted through the gain fiber or notabsorbed in it may be coupled out of the cladding by using indexmatching materials (with proper heat-sinking, if necessary) so that onlyamplified laser radiation is propagated out of the fiber LMA core.Commercially available power stripers can be used for this purpose.

A preferred configuration of a pump laser source comprises a 905-925 nmmultimode fiber coupled, TEC free InGaAs laser diodes without wavelengthstabilization. Using such multimode, wavelength stabilization freediodes as a pumping source for cladding pumping of the gain fiber iscost effective and affords power scalable for a high power Yb fiberlaser operating at a fundamental wavelength around 980 nm and withsubsequent nonlinear frequency conversion to the blue-green and UVspectral ranges. Other multimode fiber coupled diode lasers such assingle diode bar coupled or single diode laser emitter coupled laserscan also be used as a pumping source, with or without wavelengthstabilization.

In a preferred amplifier configuration as shown in FIG. 4 and as will bedescribed, a Yb-doped PM single clad or PM LMA DC gain fiber 424 may becladding pumped in the 915 nm absorption band using co- and counterpumping geometries, i.e., when the pump light is coupled into thecladding either co-propagates with the seed light travelling in thecore, counter propagates to the seed light travelling in the core orboth co-propagates and counter propagates with the seed lightsimultaneously.

FIG. 4 illustrates a second embodiment of a laser system in accordancewith the invention that effectively energy scales a 980 nm seed laser410 by Yd-doped fiber amplifiers. The output of laser 410 may beconnected, in series, to a fiber coupled isolator 412 (in addition anyoptical isolator that may be incorporated into the package of the fibercouple diode laser, i.e., between the diode laser chip and the opticalfiber), an acoustic-optical modulator (AOM) 414 which produces opticalpulses through intensity modulation, a core or cladding pumped fiberamplifier/pre-amplifier 416 to amplify low power pulses created by AOM414 to the energy level close to the gain saturation of the boosteramplifier 463, a band pass filter 418, and a second isolator 420 to a PMsignal and pump combiner 422 (WDM or fused). The DC un-doped fiber ofthe PM pump and signal combiner 422 may be connected to a DC LMAYb-doped double clad PM gain fiber 424 having an appropriately chosenlength so that it may be coiled and heat sinked. The output of theYb-doped gain fiber 424 may be coupled to another PM signal and pumpcombiner 426. The pump ports of the signal and pump combiners 422 and426 may be coupled to a plurality of fiber coupled low brightnessmultimode diode laser (DL) sources 430, 432, respectively, withoperating wavelengths near 915 nm in a co-pumped and counter pumpedconfiguration for the Yd-doped gain fiber 424, as shown. The output fromcombiner 426 is connected to a fiber clad power stripper 434 to removeany remaining part of the pump power propagating in the fiber cladding.The output end of the Yb-doped gain fiber is preferably end-capped,angle cleaved or polished as shown at 436, preferably with more than 8degrees between the polished surface and the direction of the corepropagating light. The amplified 980 nm output of the fiber may then becoupled to a collimating lens 440 and directed by mirrors 442 and 444 toanother lens 448 and to a second harmonic generator (SHG) crystal 450.The 490 nm second harmonic light from SHG crystal 450 may be directedthrough lenses 452, 454 to a fourth harmonic generator (FHG) crystal460. Another lens 462 may collimate the 245 nm fourth harmonic lightexiting the FHG crystal 460. In accordance with the invention, apowerful 980 nm DFB PM fiber coupled diode laser 410 may be temperaturetuned to the 978 nm spectral position, then spliced to the PM fiberisolator 412 which, in turn, may be spliced directly to the input signalport of the fused PM pump to signal combiner 422. A PM, LMA Yb-doped DCfiber 424 can be used as a gain medium for fiber booster amplifier 463.

In accordance with the invention a preferred laser configuration thatmay be used for high power operation is as follows. The seed laser 410may comprise a 980 nm single frequency PM fiber coupled (PM980 fiberwhich is a 6/125 PM panda fiber) diode laser temperature tuned to 978nm, coupled to the fiber coupled 980 nm PM isolator 412 followed bytapered mode field adapter with PM-6/125 fiber at the input port andPM-LMA 25/250 fiber at the output port and to the input signal port ofthe pump to signal combiner 422. Combiners 422 and 426 may each be afused PM pump to signal 3+1->1 (3×200/220, NA=0.22 pump ports and1×PM-LMA-DC-25/250 through signal port) combiner. The pump lasers 430,432 may be 915 nm diode lasers. The total pump power from the threefiber coupled pump diode lasers 430 is preferably 300 W (100 W from eachfiber coupled 915 nm diode laser 430). The Yb-doped booster amplifiergain fiber 424 may comprise approximately 50 cm-100 cm of Yb-doped DCfiber with a 25 μm core diameter, a 250 μm cladding and between 4 dB/mand 6 dB/m clad absorption at 976 nm wavelength having a 20 degreecleaved output end and coiled about an aluminum cylindrical heat sinkwith a coiling diameter between approximately 40 mm and 60 mm. Thecoiled fiber also acts as a high order waveguide spatial mode filter.With a minimum conversion efficiency of 10%, the Yb fiber laser andamplifier configuration should produce an output of over 30 W of 978 nmradiation to a 2 cm long PPLT nonlinear crystal 450 located between thetwo lenses 448, 452 for focusing and re-collimation of the laser beam.With about 0.3%/W/cm conversion efficiency of the PPLT, which shouldproduce about 18% conversion efficiency from a 30 W, 978 nm fiber laser,the system can deliver approximately 5.4 W of 489 nm blue-green laserline with subsequent nonlinear frequency conversion to the UV spectralrange.

Amplifier/pre-amplifier 416 may be an Yb-doped fiber amplifier or a sidepumped DPSS microchip amplifier. Examples, are microchip PM fibercoupled amplifiers based on Yb-doped yttrium vanadate (Yb:YVO4);Yb-doped gadolinium vanadate (Yb:GdVO4); Yb-doped potassium gadoliniumtungstate (Yb:KGW); Yb-doped potassium yttrium tungstate (Yb:KYW) Suchmicrochip amplifiers advantageously reduce cost while boosting peakpower, which is not possible to achieve using a SOA.

A fiber coupled microchip laser used as a seed laser 410 in accordancewith the invention may comprise a Nd:YVO4 (or Nd:GdVO4) crystaloptically bonded to Yb:KYW (or Yb:KGW) with a laser cavity mirror, i.e.,dichroic high reflector at about 980 nm and 912 nm (or 914 nm as in thecase of the Nd:GdVO4 crystal) and high transmitter at about 808 nmcreated on the Nd:YVO4 (or Nd:GdVO4) crystal. Another mirror for anoutput coupler may be created on the Yb:KYW (or Yb:KGW) crystal withpartial transmission at 980 nm and high reflection at 912 (or 914 nm asin the case of the Nd:GdVO4 crystal). The Nd:YVO4 (or Nd:GdVO4) crystalmay be optically pumped by a 808 nm diode laser (either internal gratingstabilized, VBG stabilized or a non-stabilized), producing intra-cavitylaser emission at 912 nm (or 914 nm as in the case of the Nd:GdVO4crystal) which circulate inside the microchip laser resonator. The Ndlaser operates a 4F3/2->4I9/2 transition with 912 nm (or 914 nm) laserwavelength and produces intra-cavity circulating power which in turnintra-cavity optically pumps the Yb:KYW (or Yb:KGW) laser crystal,producing ZPL laser emission of these laser crystals at 980 nm throughthe output coupler. In addition to the Nd:YVO4 (or NdYGdVO4) and Yb:KYW(or Yb:KGW), a nonlinear crystal, preferably quasi-phase matched (suchas PPLN-MgO) may be inserted into the microchip laser cavity throughoptical contact to the Yb:KYW (or Yb:KGW) crystal (or though freespace). The laser may produce blue-green laser power in the vicinity ofapproximately a 490 nm spectral line, in which case microchip lasercavity mirrors may be deposited on the Nd crystal and the nonlinearcrystal. A TEC can be used for thermal management of the microchiplaser.

FIG. 5 illustrates a third embodiment of the invention in which a DFB,100 mW, PM fiber coupled diode laser 510 operating at 981 nm wavelengthis used as a CW seed laser for the master oscillator. The 981 nm laseroutput may be supplied, in series, to an isolator 512, anamplifier/pre-amplifier 514, and an intensity modulator 520 which maycomprise a PM fiber coupled small chirp or chirp-free electro-opticalMach-Zhender modulator producing 1 ns pulses at a 1000 Hz pulserepetition rate (PRR) (i.e., a duty cycle of 0.000001). With a 4 dBinsertion loss, about 40 nW of the 981 nm light average output power maybe obtained from the Mach-Zhender modulator 520. The output from theintensity modulator may be applied to a band pass filter 522, anisolator 524, and to a single or multi stage Yb³⁺-doped fiber amplifier530. Elements 532, 534, 536 and 538 may be similar to elements 122, 124,126 and 128 previously described in connection with FIG. 1. Acore-pumped or a cladding pumped Yb-doped fiber amplifier 538 operatingon the zero-phonon transition line of Yb³⁺ in LMA Yb-doped fiber andacting as a last stage fiber amplifier booster may be used to boost thepulse energy to a level of 10-20 μJ (0.01 W-0.02 W of average power at a1 kHz PRR). The resulting peak power of the 981 nm pulses may be on theorder of 20 kW. Such high peak power limits further successfulamplification of the laser pulses in a Yb-doped fiber amplifier due tothe detrimental nonlinear processes previously described.

In order to boost the laser pulse energy further, the output from theYb-doped fiber amplifier 530 may be focused or collimated using anoptical system 540 and supplied to a solid-state bulk crystalline laseramplifier 544 with relatively low gain saturation energy. Instead of abulk crystalline laser amplifier, one may use a matching opticalparametric amplifier or an optical Raman amplifier (or a combinationthereof). A Yb³⁺:KYW crystal has relatively good thermo-optical,mechanical and laser properties, and is a good candidate for the solidstate bulk amplifier 544. The solid state bulk laser amplifier 544 maycomprise a single or multi-stage diode-pumped Yb³⁺:KYW amplifier pumpedby 915 nm, 450 mJ diode lasers. The amplified 981 nm laser pulses fromthe amplifier at the maximum of the zero-phonon Yb³⁺:KYW gain band mayhave a pulse energy of 45 mJ per pulse. For transform limited pulseamplification, the Yb³⁺:KYW amplifier can produce a pulse peak power of45 MW at 981 nm with less than 1 GHz spectral width (<0.002 nm). Theamplified 981 nm laser pulses from amplifier 544 may be frequencyconverted to 196.2 nm in a fifth nonlinear harmonic generation process1ω+4ω→5ω using second harmonic 546, third harmonic 548, fourth harmonic550 and fifth harmonic 552 generators. A pulse energy of about 5 mJ maybe obtained for the 196.2 nm laser pulses. Focusing of a 5 mJ laserpulse to a round spot with a spot diameter of the order of 50 μm gives apulse energy density of approximately 250 J/cm².

In another embodiment a diode laser pumped Chromium (Cr³⁺)-dopedLaSc₃(BO3)₄ (Cr³⁺:LSB) laser crystal can be used as a bulk pulse energybooster amplifier 138 (FIG. 1), 544 of the Yb-doped fiber laser MOPAoperating at 980 nm. This laser crystal has optical gain near 980 nm andcan be directly optically pumped by red diode lasers.

In fourth embodiment the invention affords pulse and CW operation of Erfiber lasers based on LMA, DC Er³⁺-doped gain fibers optically claddingpumped either resonantly into a 1530 nm absorption band or a 976 nmabsorption band of Er³⁺. The gain band of Er³⁺-doped fibers spans fromapproximately 1530 nm to over 1600 nm. The approach described above forYb-doped fiber laser may also be used for laser power scaling with closeto diffraction limited laser beam quality when an Er³⁺-doped LMA fiberis used (with appropriate selection of MO wavelength and spectralproperties of all other system components, appropriate for an Er³⁺ fiberlaser). This enables pulse and CW laser operation at virtually any laserline in the 1530-1610 nm spectral range as well as for red and UVoperation of these lasers via nonlinear frequency conversion to higherharmonics of their fundamental frequencies.

FIG. 6 illustrates the fourth embodiment of the invention that has anoverall configuration similar to that described for the third embodimentof FIG. 5. The fourth embodiment of FIG. 6 may use a fiber coupled DFBdiode laser as a seed laser 610 with an operating wavelength of 1544 nminstead of a 981 nm diode laser 510. The output of the laser 610 may becoupled to a fiber coupled isolator 612 (in addition or instead of theoptical isolator incorporated into the package of the fiber couple diodelaser, i.e., between the diode laser chip and the optical fiber) andthen injected into the fiber Er-doped pre-amplifier 614 comprising apump diode laser, a pump and signal fiber combiner (WDM or fused) andEr-doped or an Er³⁺—Yb³⁺-doped gain fiber. The light at the output ofthe pre-amplifier 614 may be then coupled to an intensity modulator 620which may be a Mach-Zhender modulator such as 520 described above, to aband pass filter 622, an isolator 624 and to a single or multi-stageEr³⁺-doped or an Er³⁺—Yb³⁺-doped fiber booster amplifier section 630.Each stage of amplifier 630 may comprise an amplifier 632, a band passfilter 634, an isolator 636 and an un-doped fiber 638. The band-passfilter 634 filters out residual ASE signal which may be present in theoutput of the fiber amplifier 632 and optical isolator 636 protects thefiber amplifier 632 from power feedback originating in the nextamplification stage. The DC un-doped output fiber of the pump and signalcombiner (which is a part of the fiber amplifier 632) may be spliced tothe DC LMA Er³⁺-doped or an Er³⁺—Yb³⁺-doped gain fiber having anappropriately chosen length (an example is PM version of LiekkiEr120-20/125 LMA DC Er³⁺-doped fiber). The pump ports of the pumpcombiner may be spliced to the fiber coupled low brightness, multimodediode lasers with oscillation wavelengths near 976 nm which radiation iscoupled into the cladding of the Er-doped DC gain fiber using pump powercombiner. The output end of the Er³⁺-doped cladding pumped gain fiber ispreferably end-capped and angle cleaved or polished with more than 8degrees between the polished surface and the direction of the corepropagating light. Fifth harmonic generation of the fundamental 1540 nmfiber laser wavelength affords a 308 nm UV laser line which correspondsto the 308 nm line of an excimer laser.

Obtaining a low core NA is much more difficult in Er-Yb-doped fibersthan in Er-doped fiber alone due to the high phosphorous concentrationrequired for pump transfer in the Yb³⁺→Er³⁺ case. Therefore, mostcommercial Er-doped fibers are either Er—Yb co-doped with a single modecore (SM or PM) and high core NA (over 0.1) or just Er-doped in a LMAstructure with core NA<0.1

Using a single laser source with a minimum number of nonlinear frequencyconversion stages (no other resonant cavities such as OPO or resonantcavity harmonic generation) in the case of an Erbium doped fiber laser,i.e., at a single laser wavelength in the range of approximately1530-1570 nm, the bulk solid-state amplifier 138 (FIG. 1) should have anopticalgain in the vicinity of 1550 nm. Most widely used highlythermally conductive laser crystals such as Er³⁺:YAG, Er³⁺:YVO₄ orEr³⁺:GdVO₄ demonstrate gain in approximately the 1.6 μm spectral rangei.e., shifted to the longer wavelength range of the optical spectrumfrom 1.5 μm or favor 2.9 μm transition of Er³⁺ (⁴I_(11/2)→⁴I_(13/2)) anddemonstrate low efficiency in 1.5 μm lines. Therefore, these crystalsare not well suited (i.e. are not spectrally matched) for the efficientamplification of a 1.5 μm laser line. This is a main reason why there isa lack of high pulse energy and high average power solid-state lasers inthe vicinity of the 1.5 μm other than bulk Er³⁺-doped glass amplifiers.These amplifiers afford high energy amplification, but because of thelow thermal conductivity of glass, they are restricted to a few hundredsof mW of average output power.

Laser sources in accordance with the invention address this in two ways.One approach is to use Er³⁺- or Er³⁺—Yb³⁺ doped crystalline lasermaterials such as Er³⁺:KYW, Er³⁺—Yb³⁺:KYW, Er³⁺:YAB or Er³⁺—Yb³⁺:YABlaser crystal which have a gain band that spans approximately 1520 nm to1600 nm with a center near approximately 1.55 μm. These crystals can bediode laser pumped with commercially available InGaAs laser diodesoperating in 905-980 nm spectral range, which are one of the mostreliable and efficient class of laser diodes.

A second approach is to use a sequence of optical amplification andnonlinear frequency conversion stages to achieve high energy, highrepetition rate and high average power UV laser light. As shown in FIG.6, the first fiber laser amplifier 614 amplifies the 1544 nm laser beamproduced by the seed laser 610. After intensity modulation by modulator620, the booster amplifier system 630 amplifies the laser pulsesfurther. The amplified laser pulses are then frequency converted usingthe SHG generator 640 to the red line at 772 nm. Second harmonicgeneration in the 1544 nm spectral range to produce 772 nm light can beachieved either using phase matched or quasi-phase matched nonlinearmaterials, and can achieve a conversion efficiency of over 50%. Next,prior to further nonlinear frequency conversion, as shown in FIG. 6, abulk solid-state amplifier 644 may be used after filtering the 1544 nmfundamental beam at 642 to boost the pulse energy of the 772 nm light.Instead of a bulk crystalline laser amplifier, one may use a matchingoptical parametric amplifier or an optical Raman amplifier (or acombination thereof), as previously described. Subsequently, theamplified 772 nm laser pulses may be nonlinearly frequency converted insecond, third and fourth harmonic generators 646, 648 and 650 to 193 nm.In the case of a laser amplifier used as a bulk optical amplifier,preferred laser crystals for the amplifier 644 are Ti³⁺: sapphire orAlexandrite. Other crystals such as Cr³⁺:LiCAF can also be used, butthey have limited output average power capability. Both of thesematerials have a maximum of the gain near 770 nm and can be laser pumpedor diode pumped. Most importantly, both Ti³⁺:sapphire and Alexandritehave high thermal conductivity and can operate in a high average powermode. Additionally, a Ti³⁺:sapphire laser amplifier can be pumped byreliable and available DPSS green lasers or by blue laser diodes. Forboth high pulse energy and high repetition rate, there are commerciallyavailable systems with pulse energy of over 30 mJ at 527 nm and pulserepletion rate of 1 kHz. A 45 mJ, 527 nm, DPSS laser pumpedTi³⁺:sapphire crystal used as a booster amplifier for 772 nm can produceat least 4 mJ of pulse energy. With subsequent frequency conversion tothe second, third and finally fourth harmonics, a laser source inaccordance with the invention can produce a pulse energy of over 0.5 mJat 193 nm. When this 0.5 mJ pulse energy is focused to a round spot withthe diameter of 0.65 mm, the energy density will be 150 mJ/cm², which iswhat most LASIK eye systems require from a laser source to be deliveredto the cornea target.

In one embodiment, seed laser 610 may be a DFB, 100 mW, PM fiber coupleddiode laser operating at 1544 nm wavelength as a CW master oscillator.The 1544 nm laser light intensity may be modulated in intensitymodulator 620 comprising a PM fiber coupled small chirp or chirp freeelectro-optical Mach-Zhender modulator producing 1 ns pulses at 1000 Hzpulse repetition rate (PRR) (i.e., duty cycle of 10⁻⁶). With a 4 dBinsertion loss, about 40 nW of the average output power may be obtainedat the output of the Mach-Zhender modulator. A core- or cladding pumpedEr-doped fiber power amplifier 630 may be used to boost the pulse energyto the level of 10-20^(μJ) (0.01 W-0.02 W of average power at 1 kHzPRR). The resulting peak power of the 1544 nm pulses will be on theorder of 20 kW. This high peak power limits further successfulamplification of the laser pulses in an Er-doped fiber amplifier due todetrimental nonlinear processes. In order to boost further the laserpulse energy, a solid-state bulk laser amplifier with relatively lowgain saturation energy is used. Ti:sapphire has a very goodthermo-optical, mechanical and laser properties and is a good candidatefor the task to boost the laser pulse energy. To convert the 1544 nmlaser wavelength to one to match the Ti:sapphire amplifier gain band,the SHG 640 may use a MgO:PPLN nonlinear crystal to convert the 1544 nmwavelength to 772 nm. With a typical conversion efficiency for >1 kWpulse peak power of 40-50%, approximately 100 mW average power at 772 nmcan be obtained. To boost the laser pulse energy the 772 nm pulses maythen be amplified in a single or multi-stage Ti:sapphire amplifierpumped by a 527 nm, 45 mJ diode-pumped Nd:YLF laser. Dual or multi-passamplification geometries can be used to increase the Ti:sapphireamplifier energy extraction efficiency. The amplified 772 nm laserpulses may reach a level of 10 mJ per pulse. For transform limited pulseamplification, the Ti:sapphire amplifier booster 644 can produce a pulsepeak power of 10 MW at 772 nm with less than 1 GHz spectral width(<0.002 nm Instead of a bulk crystalline laser amplifier to amplify the772 nm laser beam, one may use a matching optical parametric amplifier(such as 532 nm laser beam pumped BBO or KTP) or an optical Ramanamplifier, as previously described. The three harmonic generators 646,648 and 650 that convert 772 nm to 193 nm may use a set of three BBOnonlinear crystals. The 193 nm laser pulse energy will be approximately1 mJ, which when focused to a round spot diameter of 0.8 mm affords apulse energy density of about 200 mJ/cm².

In another embodiment, a core-pumped or cladding pumped Er-doped fiberpre-amplifier may be used to boost the pulse energy to the level of10-20 μJ (0.01 W-0.02 W of average power at 1 kHz PRR). Subsequently, asolid-state bulk laser amplifier with relatively low gain saturationenergy, preferably diode-pumped Er³⁺—Yb³⁺:KYW crystal having moderatethermo-optical, mechanical and laser properties may be used to boost the1544 nm laser pulse energy. The gain band of the Er³⁺—Yb³⁺:KYW crystaloverlaps the 1544 nm laser line. Also, this laser crystal can bedirectly diode-pumped with 980 nm InGaAs laser diodes. Similarly to aEr³⁺—Yb³⁺:KYW crystal, one may use a diode-pumped Er³⁺—Yb³⁺:YAB lasercrystal to boost the 1544 nm laser pulse energy within currentembodiment. Such bulk laser amplifiers can boost the fiber laser pulseenergy to the level of over 20 mJ. Subsequent nonlinear frequencyconversion of the fundamental frequency of this hybrid laser systemaffords UV laser pulses at 193 nm wavelength (8^(th) harmonic generationfrom 1544 nm fundamental beam) with overall energy conversion efficiencyover 5%. The expected pulse energy of the 193 nm laser pulses is closeto 1 mJ, and with focusing to a round spot diameter of 0.8 mm achieves apulse energy density of about 200 mJ/cm².

In a fifth embodiment shown in FIG. 7, a laser source in accordance withthe invention may use pulse and CW operation of Tm³⁺ fiber lasers basedon LMA, DC Tm³⁺-doped gain fibers optically cladding pumped eitherresonantly into the 1650 nm absorption band or the 795 nm absorptionband of Tm³⁺-doped glass. The gain band of Tm³⁺ doped fibers isapproximately 1850 nm to over 2100 nm. The approach of the fourthembodiment described above for Er-doped fiber laser may also be used forlaser power and energy scaling with close to diffraction limited laserbeam quality when an Tm³⁺-doped LMA fiber is used (with appropriateselection of MO wavelength and spectral properties of all other systemcomponents, appropriate for a Tm³⁺ fiber laser). This fifth embodimentachieves pulse and CW laser operation at virtually any laser line in the1850-2150 nm spectral range as well as operation of these lasers vianonlinear frequency conversion to higher harmonics of the fundamentalfrequencies at red, blue, green and UV wavelengths. As shown in FIG. 7,a laser source in accordance with the fifth embodiment may employ afiber coupled CW seed laser 710 with an operating wavelength of 1944 nmand spectral line width of <30 MHz. Seed laser may be coupled to a fibercoupled isolator 712, an amplifier/pre-amplifier 714, a fiber coupledelectro-optical intensity modulator 716 that produces 10 kHz opticalpulses with a 1 ns pulse width. The laser light pulses are provided to anarrow band-pass filter 718, to a fiber coupled optical isolator 720,and to a single or multistage Tm-doped PM fiber booster amplifier 730comprising a fiber amplifier 732, followed by a band-pass filter 734, anoptical isolator 736 and may also contain an un-doped fiber 738. Fiberamplifier 732 may employ as a gain fiber a cladding pumped DC LMATm³⁺-doped fiber, such as a PM Nufern LMA DC Tm³⁺-doped fiber (i.e.,PLMA-TDF-25/400). The output end of the Tm³⁺-doped cladding pumped gainfiber is preferably end-capped and angle cleaved or polished with morethan 8 degrees between the polished surface and the direction of thecore propagating light. Pulses at 1944 nm and approximately 10 μJ fromthe Tm-doped fiber laser having a 10 kHz pulse repetition rate and 1 nspulse duration may be further amplified in a diode pumped Tm³⁺:GdVO₄bulk laser amplifier 740 having its maximum gain near 1944 nm. Thus,amplifier 740 produces minimum ASE and good conversion efficiency. Thebulk amplifier may employ either single or multi-stage amplificationgeometries for stored efficient energy extraction from the bulkamplifier and for an output pulse energy level of tens of mJ. Instead ofa bulk crystalline laser amplifier to amplify 1944 nm laser beam, onemay use a matching optical parametric amplifier or an optical Ramanamplifier, as previously described.

Further nonlinear frequency conversion of the fundamental laserfrequency at 1944 nm to its 4^(th) harmonic using nonlinear opticalcrystals 742, 744 produces a 486 nm wavelength with about 1 mJ of closeto transform limited pulse energy with a narrow spectral line-width, 1ns pulses, and over 10 W of average power, which is what is required forfree space communications. This spectral line corresponds to theposition of one of the most intense Fraunhofer lines used for free spaceand underwater communications, and corresponds to the minimum absorptionof the deep water. Using high pulse energy and high average power, this486 nm laser opens up new opportunities in free space and underwatercommunications

Further nonlinear frequency conversion of the fundamental laserfrequency at 1944 nm to its 4^(th) harmonic using nonlinear opticalcrystals 742, 744 produces a 486 nm wavelength with about 1 mJ of closeto transform limited pulse energy with a narrow spectral line-width, 1ns pulses, and over 10 W of average power, which is what is required forfree space communications. This spectral line corresponds to theposition of one of the most intense Fraunhofer lines used for free spaceand underwater communications, and corresponds to the minimum absorptionof the deep water. Using high pulse energy and high average power, this486 nm laser opens up new opportunities in free space and underwatercommunications

The invention affords a way to scale the pulse energy and average outputpower of the Tm³⁺-doped blue or UV fiber lasers. It affords pulse and CWoperation of Tm³⁺ fiber lasers based on PM single clad and/or PM LMA,DCTm³⁺-doped gain fibers optically cladding pumped either resonantlyinto the 1650 nm absorption band or the 795 nm absorption band of Tm³⁺in fiber glass. The gain band of Tm³⁺ doped fibers is from approximately1850 nm to over 2100 nm.

As described, the invention may employ a Tm³⁺-fiber laser, whereexternal cavity, narrow line-width SOA based fiber coupled laseroperating in a Littrow or Littman configuration that may be used as afiber coupled CW seed laser. A CW diffraction grating based tunablefiber coupled diode laser for 2 μm spectral range, a 150 kHz spectralwidth and an operating wavelength of 1944 nm may be intensity modulatedby a fiber coupled Mach-Zhender intensity modulator, such as previouslydescribed, with a 1 ns pulse width and a 10 kHz pulse repetition rate.The output of the modulator may be amplified in a PM single clad, corepumped Tm fiber (for example a Nufern model PM-TSF-9/125) with a 1550 nmSM fiber coupled laser diode. The output of the PM SM Tm-doped fiberamplifier may be supplied to a fiber coupled optical isolator and anarrowband pass filter to prevent possible back reflection fromsubsequent amplifier stages and suppress residual ASE. The resultinglight is then injected in a PM Tm-doped fiber amplifier based on PM LMADC Tm³⁺-doped fiber (an example is PM version of Nufern LMA DCTm³⁺-doped fiber, i.e., a PLMA-TDF-25/400). The amplifier can be pumpedwith a fiber coupled diode lasers through a fused PM pump combiner. Thepump ports of the pump combiner may be spliced to the fiber coupled lowbrightness, multimode diode lasers with oscillation wavelengths near1550 nm (resonant pumping of Tm³⁺ into the 1650 nm absorption band). Thepump light is thus coupled into the cladding of the Tm-doped DC gainfiber. Multiple Tm-doped amplifier stages can be used to achieve maximumpeak power before the onset of nonlinear processes which limit furtherpower scaling. The resulting pulses may be injected into the diodepumped Tm³⁺:GdVO₄ laser crystal acting as a pulse energy amplifierbooster at 1944 nm (as shown in FIG. 7). Dual or multi-passamplification geometries can be used to increase the amplifier energyextraction efficiency. The laser pulses of the fundamental frequency arethen nonlinearly frequency converted to the higher optical harmonicsthrough SHG and FHG to wavelengths of 972 nm and 486 nm.

While the foregoing has been with reference to particular embodiments ofthe invention, it will be appreciated that changes may be made to theseembodiments without departing from the principles and spirit of theinvention, the scope of which is defined by the appended claims.

1. A laser system for generating laser emissions in the ultraviolet orvisible spectral range comprising: a master oscillator laser providinglaser emissions on a single wavelength in one of the ranges 972-985 nm,1530-1610 nm or 1800-2150 nm; a fiber laser amplifier system having aspectral gain band that encompasses the master oscillator wavelength foramplifying the laser emissions, and a bulk optical amplifier; the bulkoptical amplifier having a spectral gain band that encompasses thewavelength of the master oscillator laser emissions for amplifying thelaser emissions from the fiber laser amplifier system and for providinga further amplified output at said wavelength; and a frequencyconversion unit for converting said further amplified wavelength fromthe bulk optical amplifier to a wavelength in the ultraviolet or visiblespectral range.
 2. The laser system of claim 1, wherein the bulk opticalamplifier is matched to the master oscillator laser and fiber laseramplifier such that it has a gain band that spectrally overlaps thewavelength of the laser emissions from the fiber laser amplifier, andsuch that it is capable of producing pulses having pulse energy of atleast 0.5 mJ, average power of at least 0.005 W and pulse widths in therange of 0.1 to 10,000 nanoseconds.
 3. The laser system of claim 1,wherein said master oscillator laser wavelength is in a spectral rangeof 972-985 nm, and said pumped fiber laser power amplifier systemcomprises a gain fiber doped with Yb³⁺ that is pumped at approximately915 nm, and said bulk optical amplifier comprises an active lasercrystalline material doped with one of Yb³⁺ or Cr³⁺ or Ti⁺³:sapphire, anoptical parametric amplifier, a Raman amplifier, or a combination ofdifferent numbers and sequence thereof.
 4. The laser system of claim 3,wherein said bulk optical amplifier comprises Yb³⁺ active crystallinematerials and is pumped at approximately 940 nm.
 5. The laser system ofclaim 1, wherein said master oscillator laser wavelength is in aspectral range of 1530-1610 nm, and wherein said pumped fiber laseramplifier system comprises a gain fiber doped with Er³⁺ or Er³⁺—Yb³⁺that is pumped at approximately 915 nm, approximately 980 nm,approximately 1480 nm or approximately 1530 nm, and said bulkcrystalline laser amplifier comprises Er³⁺-doped or Er³⁺—Yb³⁺ co-dopedactive crystalline materials and is pumped at approximately 940 nm,approximately 980 nm, approximately 1480 nm or approximately 1530 nm. 6.The laser system of claim 1, wherein said master oscillator laserwavelength is in a spectral range of 1800-2150 nm, and wherein saidpumped fiber laser power amplifier system comprises a gain fiber dopedwith Tm³⁺ that is pumped at either approximately 800 nm or 1650 nm, andsaid bulk optical amplifier comprises Tm³⁺ doped, active crystallinematerials and is pumped either at approximately 800 nm or approximately1700 nm
 7. The laser system of claim 1, wherein said master oscillatorlaser wavelength is in a spectral range of 1950-2200 nm, and said pumpedfiber laser power amplifier system comprises one of a pumped gain fiberdoped with Ho³⁺ that is pumped at either approximately 1150 nm or 1900nm or a pumped gain fiber doped with Tm³⁺that is pumped at eitherapproximately 800 nm or 1650 nm, and said bulk optical amplifiercomprises Ho³⁺ doped, active crystalline materials and is pumped eitherat approximately 1150 nm or approximately 1900 nm
 8. The laser system ofclaim 1, wherein said bulk optical amplifier comprises laser activecrystalline materials selected from the group doped with trivalentrare-earth ion materials and trivalent or tetravalent transition metalsdoped materials.
 9. The fiber laser system of claim 1, wherein said bulkoptical amplifier has at least one single-pass amplifier stage, and hasa thin disk geometry to afford power scaling proportional to S/d, whereS is an area of the thin disk and d is a thickness of the thin disk. 10.The fiber laser system of claim 1, wherein said bulk optical amplifierhas at least one single-pass amplifier stage, and has a thin slabgeometry to afford power scaling proportional to w/b, where w is a widthof the thin slab and b is a thickness of the thin slab.
 11. The fiberlaser system of claim 1, wherein said bulk optical amplifier comprisescrystalline or crystalline ceramic rod, disk or slab shaped amplifierelements having un-doped bulk amplifier material diffusion bonded to oroptically contacting a largest surface of the bulk amplifier activeelement for wave-guiding and heat management within the bulk amplifier,and wherein said un-doped bulk amplifier material comprises one ofsapphire, YAG, Silicon and other semiconductor materials.
 12. The fiberlaser system of claim 1, wherein said fiber laser amplifier systemcomprises a series of one or more stages of fiber laser amplifiers eachamplifier stage being separated by an optical isolator and by a spectralbandwidth filter.
 13. The fiber laser system of claim 1, wherein saidfrequency conversion unit comprises nonlinear optical crystals selectedto generate said wavelength in the ultraviolet range through nonlinearfrequency harmonic generation or through optical frequency mixing ofoptical beams propagating through said frequency conversion unit. 14.The laser system of claim 1, wherein said a master oscillator laserprovides continuous wave laser emissions, and the system furthercomprises a controllable intensity modulator for converting saidemissions to pulses having a selectable pulse width in the range of0.1-10,000 nanoseconds and a pulse repetition rate in the range of 10 Hzto 100 MHz.
 15. The laser system of claim 1, wherein said a masteroscillator laser provides pulse laser emissions having a selectablepulse width in the range of 0.1-10,000 nanoseconds and a pulserepetition rate in the range of 10 Hz to 100 MHz.
 16. A laser system forgenerating laser emissions in the ultraviolet or visible spectral rangecomprising: a master oscillator laser operating on a single wavelengthin the infra-red spectral region; a hybrid fiber-bulk optical amplifiersystem comprising a single series connected chain of fiber amplifierscomprising Er³⁺ or Er³⁺—Yb³⁺ doped gain fiber coupled to a bulksolid-state optical amplifier for providing high energy laser pulseshaving pulse energy greater than about 0.5 mJ; and a frequencyconversion unit coupled to said bulk solid-state optical amplifier forconverting said pulses to pulses in the ultraviolet or visible spectralrange.
 17. The laser system of claim 16 further comprising a frequencyconverter for converting the wavelength from the fiber amplifiers tomatch the gain band of the bulk solid state optical amplifier.
 18. Alaser system for generating laser emissions in the ultraviolet spectralrange comprising: a master oscillator laser operating on a singlewavelength at approximately 980 nm; a power amplifier system coupled tothe output of the master oscillator laser, the power amplifier systemcomprising a single chain of one or more series connected pumpedYb⁺³-doped gain fiber amplifier stages for providing power amplifiedpulses at said wavelength of approximately 980 nm; and a frequencyconverter for converting said power amplified pulses to laser pulses ata wavelength of the order of 196 nm.
 19. The laser system of claim 18,wherein said gain fiber comprises a polarization maintaining LMA gainfiber that is coiled for spatial mode filtering and heat sinked.
 20. Thelaser system of claim 18 further comprising a bulk solid stateamplifier, located between the fiber amplifier and the frequencyconverter, comprising active laser crystalline material doped with oneof Yb³⁺ and Cr³⁺, and Ti⁺³:sapphire and being pumped at approximately940 nm, 670 nm and 500 nm, respectively, for further amplifying saidpower amplified pulses to provide high power pulses at saidapproximately a 196 nm wavelength.
 21. The laser system of claim 18further comprising a fiber pre-amplifier system between said masteroscillator laser and said power amplifier system, the fiber preamplifiersystem comprising a Yb-doped gain fiber amplifier pumped atapproximately 915 nm.
 22. The laser system of claim 18, wherein saidmaster oscillator laser outputs continuous wave laser emissions, and thesystem further comprising a controllable intensity modulator forconverting said laser emissions to pulses having a selectable pulsewidth in a range of the order of 0.1-10,000 nanoseconds and a pulserepetition rate in the range of 10 Hz to 100 MHz.
 23. A method ofgenerating laser emissions in the ultraviolet or visible spectral rangecomprising: generating laser pulses at a single wavelength in one of theranges 972-985 nm, 1530-1610 nm or 1800-2150 nm; amplifying said laserpulses in a fiber laser amplifier system having a spectral gain bandthat encompasses said wavelength of the pulses, said fiber laseramplifier system comprising a single chain of series connected gainfiber amplifiers doped with an element selected from the groupconsisting of Yb³⁺, Er³⁺, Er³⁺—Yb³⁺, Tm³⁺ and Ho³⁺; further amplifyingsaid laser pulses from said single chain of fiber amplifiers in a bulksolid-state optical amplifier to produce high power laser pulses havinga pulse energy on the order of 0.5 mJ or greater; and converting saidhigh power laser pulses to a wavelength in the ultraviolet or visiblespectral range.